The preliminary treatment process removes the debris and sandy materials from the used water. The used water arrives at the water reclamation plant (WRP) and is first lifted up to a higher elevation by pumps. With the help of gravity, the used water flows through the various treatment tanks and automated mechanical screens to remove the debris. This is followed by grit settling tanks or vortex grit chambers to settle and remove the heavier sandy materials present in the used water.

The used water, now free of debris and sandy materials, flows through very slowly across large tanks called primary clarifiers. The process allows the solid pollutants in suspension in the used water to settle to the bottom of the tanks. The settled solids known as primary sludge is collected by scrapers at the bottom of the tanks and removed regularly for treatment. On the other hand, light materials like scum, greasy materials float up to the surface of the tank, and is collected and combined with the sludge for further treatment. The top water, which contains much less pollutants in suspension, leaves the primary clarifiers for secondary treatment.

The secondary treatment comprises the aeration tanks which includes a bio-reactor and final clarifiers. The used water is mixed with a culture of micro-organism known as activated sludge in the aeration tank. The micro-organism absorbs and breaks down the organic pollutants in the used water. In order to sustain the biological activities in the aeration tanks, a certain level of dissolved oxygen has to be maintained in the used water. This is achieved by blowing air through air diffuser domes placed at the bottom of the aeration tanks to create fine air bubbles in the aeration tank. In some WRPs, mechanical surface aerators are used to stir up the used water and splash it into the air to help to dissolve the oxygen into the used water. The aeration process also helps to mix the used water with the micro-organism to promote the bio-reaction process. By the time the used water reaches the end of the aeration tanks, most of the pollutants would have been absorbed by the micro-organism. The mixture of micro-organism and the treated water is then channelled into the final clarifiers.

At the final clarifiers, the micro-organism settles to the bottom of the tanks. The clear supernatant water at the top of the tank is collected and discharged from the tanks as final effluent. The micro-organism which settles to the bottom as sludge is constantly drawn out from the final sedimentation tank. A portion of the sludge is constantly returned back into the aeration tanks to maintain a desired concentration of micro-organism in the aeration tank to sustain the optimal bio-reaction process. The excess activated sludge is sent for further treatment.

The final effluent meets the discharge standards of 20 mg/l biochemical oxygen demand (BOD) and 30 mg/l total suspended solids (TSS). Part of the final effluent is further treated to industrial water which is supplied to the industries in Jurong Island. The final effluent is also further treated using advanced membrane and reverse osmosis technologies to high grade water called NEWater. The NEWater is supplied to the industries for use in the industrial processes to conserve potable water.

Raw sludge collected from the primary sedimentation tanks and excess activated sludge from the secondary treatment process contain a high percentage of water. The water content of the sludge is reduced by using dissolved air flotation thickener or centrifuge. The thickened sludge is fed into anaerobic sludge digesters for further treatment.

In the digesters, another culture of micro-organism thriving in an oxygen-deficient environment breaks down the organic substances in the sludge. The sludge is allowed to remain in the digesters for 20 - 30 days. The digestion process converts the organic matter into biogas which contains 60 - 70% methane. The biogas is then used as fuel to power dual-fuel engine generators which helps contribute to the electricity energy required at the plant.

With the digested sludge still relatively wet, disposal is difficult. Using mechanical means such as dewatering centrifuges, water content is substantially reduced to facilitate handling and final disposal. The dewatered sludge is incinerated and the ash is disposed at Pulau Semakau Landfill.